Drongos deceive but weavers let them

The fork-tailed drongo steals many of its meals by deception, scaring away other birds and animals by imitating alarm calls. But some of those warnings are real, and at least one bird species, the sociable weaver, pays attention.

Sometimes it pays to keep a bully close, even when he’s a lying thief.

Fork-tailed drongos are the bullies in this sub-Saharan story. These birds are masterful deceivers that can mimic the warning noises of 45 or so other birds and animals, sending their victims running and letting the drongos steal a meal. Susan Milius noted in Science News earlier this year:

[W]hen the birds hustle to steal food, they mimic alarms of other species more than 40 percent of the time — and use the victim’s own species’ alarms more often than another target’s. The mimicry works. [R]esearchers played various recorded false alarms for birds called pied babblers, a frequent target of the drongos’ fraud. Mimicked alarm calls of the babblers or of another bird distracted the babblers for longer than plain drongo alarms.

It wouldn’t seem logical to keep such liars near, but that’s exactly what sociable weavers, another African bird species, do. That’s because the drongos’ alarm calls are occasionally useful. And so the two species have developed a somewhat cooperative relationship, but it’s one where the drongos are still manipulative jerks.

Bruce D. Baigrie and colleagues at the University of Cape Town in South Africa studied sociable weavers and fork-tailed drongos living in an area of the southern Kalahari. The weavers live in big colonies of 20 to 500 birds that tend to stay within a couple kilometers of their nest while looking for food. The drongos are solitary (better for thievery, I suppose) and can often be found hanging out with the weavers.

The alarm call isn’t the only sound the drongos make. They also produce a sentinel call, which says something like “all’s fine here.” And the weavers are listening to that, as well as to the alarm calls, Baigrie and colleagues report July 30 in the Proceedings of the Royal Society B. While the drongos are singing “everything is OK,” the weavers let down their guard, increasing their efforts to get food and decreasing their own vigilance. And that works, because the drongos are there keeping watch. Not all their alarm calls are fake — some are real, and reliable. The weavers can take advantage of that.

After the fake alarm calls, though, the drongos are quick to start the sentinel calls, calling out the “all’s clear” message to the weavers. That lets them get back to foraging faster — which means they collect more food for the drongos to steal, say the researchers, who confirmed their observations with experiments in which they played recorded drongo calls for the weavers.

Imagine how this would work in the human world: Stick close to the bully who steals your lunch money because he’ll warn you when the bully who breaks your arm is coming? I think this relationship is best left to the birds.